1. Field of the Invention
The present invention relates to a switching power supply and, more specifically, relates to the pulse width modulation (PWM) of the switching power supply.
2. Background of the Invention
The PWM controller is an integrated circuit used in the switching power supply to control and regulate the switching duty cycle. Being subject to environmental regulations, the power system design of computers and other electrical products have been required to meet the power management and energy conservation standards. The power management is to manage the system to consume power during operation and only very little power is consumed during the non-operation mode. For the power management application in the power supply, how to save the power in light load and no load condition is a main concern. One object of the PWM modulator is to optimize the saving of the power consumption and to reduce acoustic noise when the oscillation frequency falls into the audio band.
FIG. 1 shows a typical flyback power supply circuit, in which a PWM controller 100 controls and regulates the output power. The PWM-control starts when the power supply is turned on. A capacitor 220 is charged via a resistor 210 until the supply voltage VCC of the PWM controller 100 reaches the start-threshold. Then the PWM controller 100 starts to output PWM signal and drive the entire power supply. After starting-up, the supply voltage VCC is provided from the auxiliary bias winding of a transformer 400 through a rectifier 230. A resistor 240 converts the switching current information of the transformer 400 into voltage signal for PWM control and over-power protection. Once the auxiliary voltage of the transformer 400 cannot provide sufficient power for the supply voltage VCC, the PWM controller 100 will be turned off as the supply voltage VCC is lower than the stop-threshold. The feedback voltage VFB is derived from the output of an optical-coupler 250. The input of the optical-coupler 250 is connected to the output of the power supply Vo through a resistor 290 and a zener diode 280 to form the feedback loop. Through the control of the feedback loop, the voltage VFB controls the on-time TON duration of the PWM signal through the PWM controller 100 and dominantly decides the output power.
The power loss of the power supply is an important concern. Those major losses including the transformer core loss, the transistor switching loss, and the snubber power loss are proportional to the switching frequency F. The switching period T is the inverse of the switching frequency F, T=1/F. Increasing the switching period T reduces the power loss. However, to prevent from the saturation of the transformer and shrink the size of the power supply, a shorter on-time TON is required. The saturation of magnetic components such as inductors and transformers may result without the limitation of the TON duration and will cause over-stress damage to the switching devices such as the transistors and rectifiers. Although the power consumption of the power supply will reduce in response to the decrease of the switching frequency F, an audio noise will be created when the switching frequency falls into the audio band (such as 200 Hz 8 KHz) in light load and no load conditions. Another object is to reduce the acoustic noise when the switching frequency falls into the audio band in light load and no load conditions.
Some methods had been disclosed in prior arts to increase the regulator efficiency such as varying the switching frequency and entering the xe2x80x9cpulse-skippingxe2x80x9d mode according to load conditions. For instance, U.S. Pat. No. 6,100,675, xe2x80x9cSWITCHING REGULATOR CAPABLE OF INCREASING REGULATOR EFFICIENCY UNDER LIGHT LOADxe2x80x9d (incorporated herein by reference) disclosed an oscillation frequency control circuit capable of varying an oscillation frequency of the oscillator circuit in response to load conditions. Another method is disclosed in U.S. Pat. No. 6,366,070 B1, xe2x80x9cSWITCHING VOLTAGE REGULATOR WITH DUAL MODULATION CONTROL SCHEMExe2x80x9d, which disclosed the regulator employs three operation modes, which operate at constant switching frequency for heavy load condition, use dual modulation control scheme for moderate load condition, and entering xe2x80x9cpulse-skippingxe2x80x9d for light load condition. The disadvantage of foregoing prior arts are: (1) Varying the switching frequency without the limitation of maximum on-time may result in saturation of magnetic components and cause over-stress damage to switching devices such as transistors and rectifiers; (2) The modulation of switching frequency is only controlled by the load condition and not correlated with the supply voltage. As the switching frequency is reduced too low for saving more power in light load and no load conditions, the auxiliary bias winding of the transformer or inductor might be unable to provide sufficient power for the supply voltage of the PWM controller. The PWM control may work improperly under such condition. Thus, to correlate the frequency modulation with both load conditions and the supply voltage is needed; (3) In light load and no load conditions, the switching frequency might be decreased to the audio band. If the magnetic components are not well impregnated, the audio band switching frequency might generate an undesirable acoustic noise.
To prevent the above shortcomings of prior arts, there exists a need for a better and noiseless apparatus for improving the efficiency and saving the power consumption in light load and no load conditions.
The present invention provides an adaptive off-time modulation for saving power and reducing acoustic noise. The off-time modulation is achieved by moderating a bias current of an oscillator in the PWM controller. The maximum on-time of the PWM signal is kept as a constant. Reducing the bias current increases the off-time of switching period and thus the switching period is extended. The feedback voltage derived from the voltage feedback loop and the supply voltage are taken as variables to correlate with the off-time modulation. The bias current is modulated as a function of the feedback voltage and the supply voltage. A threshold voltage is a constant that defines the level of light load. A limit voltage defines the low-level of the supply voltage. A first differential signal is generated by subtracting the threshold voltage from the feedback voltage. A second differential signal is generated by subtracting the attenuated supply voltage from the limit voltage. The sum of the first differential signal and the second differential signal is converted into the bias current. The bias current is clamped by a limiter to set up the minimum switching period in normal load and full load conditions. Once the feedback voltage is decreased lower than the threshold voltage, the bias current is reduced and the off-time of the switching period is extended continuously. When the supply voltage is lower than the limit voltage, the bias current is increased and a maximum off-time of the switching period is determined.
A control circuit provides two entrance voltages. A reference resistor transfers the reference current derived from the bias current into a voltage signal to the input of the control circuit. A first entrance voltage determines a level of audio switching frequency. As the switching frequency falls into the audio band in lightload and no load conditions, the control circuit will output an OFF signal to turn off the oscillator of the PWM controller. A second entrance voltage determines a level of starting up the oscillator of the PWM controller. Once the supply voltage is decreased or the feedback voltage is increased and the input voltage of the control circuit is greater than the second entrance voltage, the oscillator will restart working again.
Advantageously, the adaptive off-time modulation improves the efficiency and saves the power consumption of the power supply in light load and no load conditions. Meanwhile, the control circuit applied in the present invention turns off the oscillator as the switching frequency falls into the audio band, which greatly reduced the acoustic noise.
It is to be understood that both the foregoing general descriptions and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.